The invention relates to an arrangement for automatic-range finding, particularly for sharply focusing a motor-driven objective lens.
Modern cameras furnish automatic exposure measurement devices and facilities for setting the diaphragm and/or the exposure time. However, no camera on the market successfully furnishes sharp automatic focusing.
Automatic range finding methods are known, but implementing them is so costly that they are completely unsuitable for cameras. One known automatic objective focusing method involves determining the range by means of radar or optical goniometry and setting the objective to the determined range by a motor. Apart from the fact that the costs of the equipment are rather high, automatic and continuous check of the definition achieved is generally not possible.
This last mentioned drawback is avoided in a recently developed special optical system. Here the signal controlling the motor drive is generated by means of a photocell which receives light deflected by a mirror from the optical path of the objective. In this arrangement an annular reflecting mirror deflects the marginal rays of a special objective onto an axially oscillating photocell. During the oscillation of the cell through the range of maximum image sharpness, the voltage generated by the photocell in dependence on the light density passes through a flat maximum. To obtain automatic readjustment of the definition the position of the maximum is used by a phase detector which receives a reference signal in the form of the a-c voltage required for the mechanical oscillating drive.
But this method can only be used with an objective where the front and rear elements are arranged sufficiently far from each other to provide room between them for the above mentioned annular reflecting mirror. The latter only deflects marginal rays with extremely low depth of focus, to the photocell. In the photocell, these low depth of focus marginal rays cause a light density which traverses a flat maximum in dependence on the image sharpness, hence with the photocell moving in the direction of the rays.
The mechanism for generating the oscillating movement of the photocell is costly and susceptible to trouble and makes the use of complicated phase detectors etc., an absolute necessity. However, this solution is not applicable to usually existing objectives where close front and rear elements constitute parts of an assembled objective.
An object of this invention is to avoid these shortcomings.
Still another object is to improve automatic focusing systems.
Another object of the present invention is to provide an arrangement for automatic focus detection where commercial objectives can be used and which can be realized with a minimum of circuit and material expenditure.